A casting apparatus has hitherto been known which comprises a retaining furnace of a hermetical structure for a molten material, from which is extended a supplying conduit for pneumatically injecting the molten material at a low pressure directly into a cavity of a mold.
FIG. 1 illustrates a construction of a conventional casting apparatus of a pneumatic controlling system, in which reference 10 stands for a retaining furnace of a molten material, which is hermetically closed by a cover plate 12. The cover plate 12 at its central portion is sealingly traversed by a supplying conduit 14 for the molten material, which communicates with an air pressure system 16. The air pressure system 16 introduces a pressure air from a required air source through an air pressure controlling device 20 into the furnace 10. The supplying conduit 14, on the other hand, communicates through a fixed die plate 22 with a passage 26 for the molten material into a lower mold 24 which at its upper part is joined with an upper mold 28. Between the lower and the upper molds 24, 28 is arranged a cavity 30. Furthermore, a portion of the conduit 14, which is extended externally from the cover plate 12, is provided circumferentially with a gas burner 32 for preventing the molten material from solidifying in the supplying conduit 14 until the molten material poured into the cavity 30 of the mold is solidified therein.
In the conventional casting apparatus of low pressure thus constructed, the controlled air pressure is supplied from the air pressure system 16 to the hermetical retaining furnace 10, while the injection pressure of the molten material from the conduit 14 into the cavity 30 is kept constant for casting. Since the air pressure from the air pressure system 16 is released after the casting cycle is completed, the molten material in the supplying conduit 14 flows back into the retaining furnace 10, so that the level B in the conduit 14 may be same as the level A in the furnace 10.
In accordance with such conventional casting apparatus, the air pressure to the furnace 10 is controlled to maintain the constant injection pressure, resulting in a delayed response to the injection control of the molten material and a disadvantageously increased cycle time. Furthermore, in case of a programming control of the injection pressure of the molten material into the cavity 30, the same reason may cause reduced controllability and impossibility of fine control. Consequently, when a product of a complex shape should be molded, the molten material cannot adequately flow into extreme ends of the cavity, resulting in a defective product.
At each casting cycle the molten material in the conduit 14 may vary its surface level B through switching control of the air pressure, thereby to entrap air into the molten material in the conduit 14 and thus to produce a defective product having voids.
Furthermore, in the conventional casting apparatus, as shown in FIG. 1, a single conduit is arranged under the center of the mold and communicates with the retaining furnace for injecting the molten material from a single injection opening. As a result, the molten material cannot reach extreme ends of the cavity for the complex-shaped mold, resulting in increased production of defective articles.